This is kind of weird, because the formal meaning of the term “epidemic” clearly does not apply here. I feel uncomfortable going against public health officials in what is clearly their area of expertise rather than my own, but everything I’ve ever read about the official definition of the word “epidemic” requires it to be an infectious disease. You can’t “catch” obesity. Hanging out with people who are obese may slightly raise your risk of obesity, but not in the way that hanging out with people with influenza gives you influenza. It’s not caused by bacteria or viruses. Eating food touched by a fat person won’t cause you to catch the fat. Therefore, whatever else it is, this is not an epidemic. (I guess sometimes we use the term more metaphorically, “an epidemic of bankruptcies” or an “epidemic of video game consumption”; but I feel like the WHO and CDC of all people should be more careful.)

Indeed, before we decide what exactly this is, I think we should first ask ourselves a deeper question: What do we mean by “obesity”?

Anyone who has studied dimensional analysis should immediately see a problem here: That isn’t a unit of density. It’s a unit of… density-length? If you take the exact same individual and scale them up by 10%, their BMI will increase by 10%. Do we really intend to say that simply being larger makes you obese, for the exact same ratios of muscle, fat, and bone?

Because of this, the taller you are, the more likely your BMI is going to register as “obese”, holding constant your actual level of health and fitness. And worldwide, average height has been increasing. This isn’t enough to account for the entire trend in rising BMI, but it reduces it substantially; average height has increased by about 10% since the 1950s, which is enough to raise our average BMI by about 2 points of the 5-point observed increase.

And of course BMI doesn’t say anything about your actual ratios of fat and muscle; all it says is how many total kilograms are in your body. As a result, there is a systematic bias against athletesin the calculation of BMI—and any health measure that is biased against athletes is clearly doing something wrong. All those doctors telling us to exercise more may not realize it, but if we actually took their advice, our BMIs would very likely get higher, not lower—especially for men, especially for strength-building exercise.

To compensate for this, it seems like the most sensible methodology would be to figure out empirically what sort of weight is most strongly correlated with good health and long lifespan—what BMI maximizes your expected QALY.

You might think that this is what public health officials did when defining what is currently categorized as “normal weight”—but you would be wrong. They used social norms and general intuition, and as a result, our standards for “normal weight” are systematically miscalibrated.

In fact, the empirical evidence is quite clear: The people with the highest expected QALY are those who are classified as “overweight”, with BMI between 25 and 30. Those of “normal weight” (20 to 25) fare slightly worse, followed by those classified as “obese class I” (30 to 35)—but we don’t actually see large effects until either “underweight” (18.5-20) or “obese class II” (35 to 40). And the really severe drops in life and health expectancy don’t happen until “obese class III” (>40); and we see the same severe drops at “very underweight” (<18.5).
With that in mind, consider that the global average BMI increased from 21.7 in men and 21.4 in women in 1975 to 24.2 in men and 24.4 in women in 2014. That is, the world average increased from the low end of “normal weight” which is actually too light, to the high end of “normal weight” which is probably optimal. The global prevalence of “morbid obesity”, the kind that actually has severely detrimental effects on health, is only 0.64% in men and 1.6% in men. Even including “severe obesity”, the kind that has a noticeable but not dramatic effect on health, is only 2.3% in men and 5.0% in women. That’s your epidemic? Reporting often says things like “2/3 of American adults are overweight or obese”; but all that “overweight” proportion should be utterly disregarded, since it is beneficial to health. The actual prevalence of obesity in the US—even including class I obesity which is not very harmful—is less than 40%.

There’s some evidence that within the set of rich, highly-developed countries, obesity rates are positively correlated with lower life expectancy, but these effects are much smaller than the effects of high development itself. Going from the highest obesity in the world (the US, of course) to the lowest among all highly-developed countries (Japan) requires reducing the obesity rate by 34 percentage points but only increases life expectancy by about 5 years. You’d get the same increase by raising overall economic development from the level of Turkey to the level of Greece, about 10 points on the 100-point HDI scale.

Now, am I saying that we should all be 400 pounds? No, there does come a point where excess weight is clearly detrimental to health. But this threshold is considerably higher than you have probably been led to believe. If you are 15 or 20 pounds “overweight” by what our society (or even your doctor!) tells you, you are probably actually at the optimal weight for your body type. If you are 30 or 40 pounds “overweight”, you may want to try to lose some weight, but don’t make yourself suffer to achieve it. Only if you are 50 pounds or more “overweight” should you really be considering drastic action. If you do try to lose weight, be realistic about your goal: Losing 5% to 10% of your initial weight is a roaring success.

But why am I complaining about this, anyway? Even if we cause some people to worry more about eating less than is strictly necessary, what’s the harm in that? At least we’re getting people to exercise, and Thanksgiving was already ruined by politics anyway.

Well, here’s the thing: I don’t think this obesity panic is actually making us any less obese.

If we want to actually reduce obesity—and this makes sense, at least for the upper-tail obesity of BMI above 35—then we should be looking at what sort of interventions are actually effective at doing that. Medicine has an important role to play of course, but I actually think economics might be stronger here (though I suppose I would, wouldn’t I?).

Number 3: Find policies to promote exercise. Despite its small effects on weight loss, exercise has enormous effects on health. Indeed, the fact that people who successfully lose weight show long-term benefits even if they put the weight back on suggests to me that really what they gained was a habit of exercise. We need to find ways to integrate exercise into our daily lives more. The one big thing that our ancestors did do better than we do is constantly exercise—be it hunting, gathering, or farming. Standing desks and treadmill desks may seem weird, but there is evidence that they actually improve health. Right now they are quite expensive, so most people don’t buy them. If we subsidized them, they would be cheaper; if they were cheaper, more people would buy them; if more people bought them, they would seem less weird. Eventually, it could become normative to walk on a treadmill while you work and sitting might seem weird. Even a quite large subsidy could be worthwhile: say we had to spend $500 per person per year to buy every single adult a treadmill desk each year. That comes to about $80 billion per year, which is less than one fourth what we’re currently spending on diabetes or heart disease, so we’d break even if we simply managed to reduce those two conditions by 13%. Add in all the other benefits for depression, chronic pain, sleep, sexual function, and so on, and the quality of life improvement could be quite substantial.